a) Field of the Invention
The invention is directed to a process and arrangements for photothermal spectroscopy (thermal wave analysis) based on the single-beam method with double modulation technique. It is applied for measuring geometrical, thermal, electronic and elastomechanical material parameters of surface coats or layers by evaluating the photothermal response signals from areas of solid bodies close to the surface. The noncontact and nondestructive process according to the invention is applied chiefly as a test method for quality control in coating technology.
b) Background Art
Methods of photothermal spectroscopy for noncontact, nondestructive detection of parameters of thin layers are known. The physical principles and fundamentals are compiled and described e.g. in "Photoacoustic and Thermal Wave Phenomena in Semiconductors" A MANDELIS (Ed.), North Holland, N.Y. 1987. In a known method according to ROSENCWAIG, a periodically intensity-modulated pump laser produces a photothermal response in the layer which in turn locally modulates the refractive index so that the modulated optical reflection (MOR) can be measured with a so-called probe laser beam (U.S. Pat. No. 4,579,463). This method accordingly requires two lasers with different wavelengths and a precise alignment of the two beams relative to one another on the sample. In addition to the optical precision of the beam adjustment mentioned above, the solutions suggested in U.S. Pat. No. 4,634,290, U.S. Pat. No. 4,636,088, and EP 0 291 276 involve considerable expense for optical elements for adapting the thickness of the two beam bundles. Further, the inherent noise of the probe laser represents a limiting factor for the resolution capability requiring the use of lasers with extensive noise stabilization.
Single-beam methods are also known for measuring the MOR (CHEN et al., Appl. Phys. Lett. 50 (1984) 1349; A. LORINCZ, L. ANDOR, Appl. Phys. B 47 (1988) 35; M. WAGNER, H. D. GEILER, Meas. Sci. Technol. 2 (1991) 1088). To separate the MOR from the reflected modulated pump intensity, LORINCZ makes use of the fact that the photothermal response causes harmonic waves (second harmonic) in the reflected component which can be detected by lock-in detection. However, the requisite absence of harmonic waves of 10.sup.-7 cannot be achieved because of the finite nonlinearities of the modulator. This is also true with respect to compensating for noise. In the method used by WAGNER, this disadvantage is overcome by applying double modulation technique in which two modulation frequencies are impressed on the pump beam and sum or difference frequencies produced by the photothermal refractive index modulation in the sample are detected. However, it is only possible to generate the double-modulated beam without intermodulation by combining two separately modulated partial beams. Beyond the extra expenditure on optics for aligning the partial beams, this requires the use of two separate optical modulation systems. In particular, the return of the modulated partial beam components into the laser and accordingly internal modulation of the latter must be prevented. This requires expensive optical insulators.